Acroneuria evoluta: the Rivanna River at Crofton

These brown stoneflies were fairly common today at the Rivanna River at Crofton.  I picked up about 10 to look at for photos, and while I can't say for sure that they were all the same species, all three that I photographed were Acroneuria evoluta.   Here's Beaty's description.  "dorsum of head with interrupted M-shaped head pattern, appearing as a transverse row of 3 light spots in front of anterior ocellus; abdomen not banded; anal gills present.  Uncommon. ... Nymphs more common during summer thru late fall." ("The Plecoptera of North Carolina," version 3.3 [2011], p. 14)  The assigned TV is 1.7.  (For more on this ID, see the entries posted on 6/12/15 and 6/20/15.)

You can see the lack of abdominal banding in the photo above: and you can almost make out the 3 dots on the top of the head.  For the anal gills, let's draw in a little bit closer.

Another example

Actually, the 3 dots on the head might show up better on this very small nymph that I found (9 mm vs. 15 mm for the nymph at the very top of the page).

And the gills,

A. evoluta can be confused with the "brown" form of A. abnormis, the nymph that we looked at last week,

and I'm pretty sure that I've seen both nymphs at this site.  Both lack banding on the abdominal terga; both are brown.  But, remember that A. abnormis "brown" nymphs lack anal gills, and there is no "M-pattern" on the top of the head, not even light dots.

The Rivanna River at Crofton is the only site where I've found this species of common stone (Perlidae).  I've not seen it at Darden Towe Park in Charlottesville -- not yet anyway -- where the brown A. abnormis is common.    Moreover, it's a species that is not attested in the state of Virginia according to "NatureServe Explore" (http://explorer.natureserve.org/servlet/NatureServe?searchSciOrCommonName=Acroneuria+evoluta&x=0&y=0).  I've sent them my data, but so far I've been ignored.  Alas, the fate of the amateur entomologist!
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I didn't see much else on the rocks in the Rivanna this morning, though I did see a few of these nymphs, Agnetina annulipes,

and there are still some damselfly nymphs kicking around

This nymph is a "narrow-winged" damselfly -- Coenagrionidae, genus Argia.
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And this is what the Rivanna looks like at Crofton.

Habitat, species variation, and tolerance values: the common stonefly Acroneuria abnormis

I have written before about the issue of habitat and species variation -- in some detail on 8/26/15.  Let me remind you of two examples.

First, we have two Isoperla holochloras, one light and one dark.  Light,

and dark.

The light we find in all sorts of streams that vary in quality from Buck Mt. Creek, a stream that flows through pasture land, to the Rapidan River in the mountains at the entrance to the Shenandoah National Park (Graves Mill).

Buck Mt. Creek

the Rapidan River

But the dark form is one that I've only seen in the mountains.  In the Rapidan River, where it co-occurs with the light, and in a very clean tributary to the Rapidan River, the Staunton.

In fact, in the Staunton I've seen nothing but the dark nymphs.

Another case that I've mentioned before, Isoperla orata.  We have the "classic" form that we've found in the Rapidan River and in the small, pristine streams of Sugar Hollow,

and a variant form

which is also found in the Rapidan River, but I've seen it as well in the Doyles River and Buck Mt.
Creek, again, streams that are a step down in quality terms.

It seems to me that we have the same situation with the common stonefly, Acroneuria abnormis. A reminder of Beaty's species description.  "A. abnormis ... : dorsum of head with a well defined M-shaped head pattern, sometimes with interruptions; posterior margins of abdominal tergites light, dark tergal bands irregular; or dorsum of head without M-shaped head pattern and abdomen uniformly brown; anal gills always absent."  He adds "widespread and nymphs occur year round." 

I'd like to suggest that again, habitat is relevant in determining where these two forms are found.  The first, with the well defined M-pattern and the banded terga

is indeed widespread, and I see it in almost all of the streams I visit.   But in the Rivanna, we find the "brown" form (no M-pattern, and terga completely brown), a form that, so far, I've not seen anywhere else.

 (Not sure if the first type co-occurs in the Rivanna.  Something I have to look into.)  All of this makes me wonder about species variation and tolerance values.  It has to be true that the "brown" form of A. abnormis is more tolerant than the typical pattern we see, the same being true for the two types of I. holochlora and I. orata.  The NC list of tolerance values actually does distinguish between the light and dark holochloras: the former has a TV of 0.7, the latter is 0.0.  Makes very good sense.  But no distinction is made for our other two taxa.  There's only one value for I. orata -- 0.0 -- and the same is true for A. abnormis -- 2.1.   I think that there too, a distinction has to be made.

Have to contact Beaty on this one.  Could well be that this is one of those things that needs some revision.

It's Acroneuria lycorias -- not carolinensis -- at Buck Mt. Creek

Kind of messed up today with my photos.  Somehow, I had my camera set to "Aperture priority" mode (Av) -- instead of the "Shutter priority" mode (Tv) that I normally use.  Unfortunately, I had not chosen an aperture setting, so all of my photos were overexposed.  Alas!  As you can see in the photo above, by editing we can make a partial correction -- but the shading doesn't look right.

Anyway, I was curious to see what this common stonefly -- an Acroneuria nymph for sure -- was in terms of the species.   You may recall that there are two species of common stone that look almost exactly alike: A. carolinensis and A. lycorias.   (See the entry posted on 12/11/14.)  This is A. carolinensis from another stream.

While I see this one a lot in the mountain streams that I visit -- upper Doyles River, Rapidan River, north fork of the Moormans --  in 2013 I realized that in Buck Mt. Creek, I was finding A. lycorias instead.  This one.  (photo taken on 10/20/13)

 (A. carolinensis, by the way, has a TV of 1.2; A. lycorias, 2.1.)  How do we tell them apart?  There are two ways in which they're distinct: 1) A. lycorias has anal gills; A.carolinensis does not: and 2) A. carolinensis has faint, medial, longitudinal markings on some or all of the terga, on A. lycorias such markings are missing.

If you look again at the A. lycorias photo directly above, you can see both of those features.  For contrast, here's a picture of another A. carolinensis.

So which one did we find this morning?  Medial longitudinal markings seem to be absent.

I was hoping for another A. lycorias nymph; so, so far so good.  But are anal gills present?  I couldn't see them.  So I brought this nymph home and used my microscope to get really close.  And voila!


Sure enough, there they are.
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Back to the Rivanna tomorrow.  I've already corrected my camera settings!

Fall Stoneflies in the Rivanna: Two things I "hoped" to find; two things I "expected" to find -- and a surprise netspinner

The common stonefly (Perlidae), Agnetina flavescens (Midwestern Stone).  This is a species that I've only seen once before.  That was on 11/9/13: the Rivanna River at Darden Towe Park (Charlottesville), the same place I went to today.  And I was hoping to see it.  Agnetina nymphs have a setal row on the occiput, and anal gills are present.  A. flavescens is described by Beaty in the following way: "head pattern roughly M-shaped with arms directed posterolaterally and almost interrupted; a light triangular pale area between lateral ocelli; dorsum of abdomen banded, with dark bands on anterior half of segment...apex of tergite 10 light with narrow dark pigment band interrupted mesally." ("The Plecoptera of North Carolina," p. 15)

We can see all of those features on this tiny nymph save one: the anal gills.

The anal gills require a microscope view.

There is also a feature not noted by Beaty in his description which seems to be characteristic of A. flavescens: terga 8 and 9 are both pale with a single dark spot in the middle of 8 and three dark spots in 9.

While A. flavescens is not something I commonly see in the Rivanna, A. annulipes (Southern Stone) is par for the course at this time of year, and I found that species as well.

Much darker than A. flavescens, and the apex of terga 10 on this species is totally dark.  Again, we have to go to the microscope if we want to see anal gills.

On the species of Agnetina, Beaty notes: "Nymphs occur primarily in the Mountains though can be found in the Piedmont.  Uncommon."  Interesting.  A. annulipes, at least, is present in pretty good numbers in the Rivanna.  (A. capitata, by the way, is uncommon.  I've only seen that one at the Rapidan River.)
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While I was pleasantly surprised to see both Agnetina that are found in this river, I was not at all surprised to see two other stones.  The first, the "brown form" of Acroneuria abnormis.

The abnormis nymphs that we normally see have a very clear pale yellow "M" on the head, and the terga are banded.  But with this type of abnormis, the head lacks that "M" pattern, and, as Beaty notes, the "abdomen [is] uniformly brown." (p.15)  It was a young one -- note the posterior edges of the wing pads -- but it was already a pretty big nymph: 17 mm.  By contrast, the A. flavescens nymph was only 8 mm while the A. annulipes was a mere 6!

Also around -- quite a few of them, actually -- the Giant stoneflies Pteronarcys dorsata.

This is the only "Giant" that I see in the Rivanna, and it's the most tolerant Giant species with a TV of 2.4.  It's easily distinguished by a number of features.  1) the lateral angles of the pronotum are "produced"; 2) there are longitudinal stripes on the abdomen (usually 3-5, only 2 visible here); and 3) there are no lateral projections/hooks on the abdominal segments.  (See Beaty, "The Plecoptera of North Carolina, p. 28)

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It was a good day for stoneflies at the Rivanna, lots of them crawling around in the leafpacks.  I saw very few mayflies, and very few netspinner larvae in a stream that's filled with them in the summer.  But I did pick up one to ID.

I was completely shocked when I looked at this through the camera.  It's a Macrostemum!  How do we know?  The "carinate, flattened head".  (Beaty, "The Trichoptera of North Carolina," p. 77)  Let's have a close-up look.

You can see the carina -- the raised ridge circling the top of the head -- in this photo, and if you can't tell how flat that head is, here's another angle to use.

And with these gills, these larvae should have no problem getting oxygen out of the river!

Species?  Don't know.  There are two possibilites: M. carolina or M. zebratum.  On both, the head is "reddish brown".  (Beaty, p. 77)  The difference -- the size of the tubercle near the eye.  This.

On carolina it's "large"; on zebratum it's small.  I'd have to see both species to know which one I've found.  This is a common netspinner that I always see in the North Fork of the Rivanna in summer, but this is the first time I've picked one up in the main river.  While this one is brown, those in the North Fork have always been green.  (Photo from 7/16/11)

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It was a pleasure to get back out to our streams.  The Rivanna will be my destination for the foreseeable future.  We're in semi-drought conditions: most of our streams are dreadfully low and warm.

Very strange things in the water: a common netspinner pupa

No, it is not a baby chicken.  It's a common netspinner pupa, Hydropsyche/Ceratopsyche sp.  And it's been so long that I've seen one that I wasn't sure what it was until I got close enough to take pictures. We shouldn't be surprised.  We see the larvae all winter long, and I guess it's time for them to hatch.  This one had just broken its way out of its pupal dome and was -- I assume -- on its way to the surface.

Like all "retreat-making" caddisflies, the common netspinner builds a dome out of pebbles when it's ready to pupate.  The cocoon is encased in the dome which is then attached to a rock.  The process is described in detail -- with illustrations -- by Wiggins on pp. 62-64 of his wonderful book, Caddisflies: The Underwater Architects (Toronto, 2004).  (Also, see the entry posted on 12/24/11.)

But how do we establish the genus ID?  The only source I have for making pupa ID is the comprehensive An Introduction to the Aquatic Insects of North America (4th edition, 2008), which is edited by R.W. Merritt, K.W. Cummins, and M.B. Berg.  The Hydropsychidae pupa is described on p. 460, and Hydropsyche sp. is illustrated on p. 467.   The key to the ID is the number of "hook plates" on the abdominal segments (dorsal view).  I quote: "Abdominal segments III and IV each with two pairs of hook plates; segment V with only one pair of hook plates."  And that's what we've got.

How about the species?  I have no way of knowing for sure.  But, I've only seen 3 netspinner species in this river -- the Rapidan -- Ceratopsyche/Hydropsyche sparna, Ceratopsyche/Hydropsyche alhedra, and Ceratopsyche/Hydropsyche slossonae.

In order...

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Not the only strange thing that I found yesterday.  I collected a lot of these unwelcomed diptera:  Tabanidae, horse flies!

Nymphal growth, instars, and frequency of molting

Yesterday, I went to the lower Doyles River -- shortly before it enters the Moormans.  While I didn't see much of interest, I did find -- as I often do at this time of year -- a Perlid nymph that had recently molted.  At this stage, early in a new instar, it has not yet sclerotized (darkened with patterns); it has just shed its previous exoskeleton, and the cuticle is in the process of hardening.  When that happens it will look something like this (a second young nymph that I found yesterday).

While I see freshly molted nymphs at various times of the year, if memory serves, I see them more frequently at this time of year -- September through December.  (E.g. here's one that I found on September 26, 2012.)

Why?  Is it possible that growth is more rapid during late summer and fall?  Rapid growth would mean a shorter time between instars, and that would increase the odds of seeing this kind of nymph.

Last night I dug around to see what I could find in the internet sources, and I suspect this conclusion is true.  The key thing -- stonefly nymphs grow rapidly throughout the fall then slow down in the winter.  Data supporting this fact was compiled by George L. Vaught in his M.A. thesis, "The History and Ecology of the Stonefly Neoperla Clymene." (1974, North Texas State University)

In his thesis Vaught notes early on (introductory summary of conclusions), "Nymphs required one year of development from the egg stage to adult, water temperature was the main factor influencing seasonal growth."  (emphasis added by me)  Obviously, the water temperatures are warm in the fall, cool to cold in the winter, them warm up again in the spring.   In line with these temperature changes, "Most early nymphs begin appearing late in the summer from the June-July adult peak emergence and oviposition.  A short spurt of fall growth is experienced before a period of slow size-increase sets in through the winter."  "Growth is resumed in March and continues until emergence." (p. 19)

Vaught tracks growth by looking at two different things: 1) wing-pad length (Fig. 5, p. 23), and 2) changes in head-capsule width (Fig. 6, p. 25)  If I'm reading his data correctly, the mean changes in wing-pad length (Fig.5) go from .5 mm to .62 mm from September to October and they jump again from November through December, but they drop to .61 to .62 from December to January.  The same is true for changes in the head capsule width (Fig. 6).  The mean width change from September to October is from .9 mm to 1.16 mm but from December to January it's only 1.15 mm to 1.16 mm.
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Interesting.  Sort of fun to know these things.  Oh, and one other thing, just in case you've been wondering how many instars are involved in these changes, Vaught notes for the Neoperla nymphs that he studied,  "Nymphs underwent up to 23 instars (males from 18 to 20, and females from 20 to 23 instars)."  That's a lot of changes, and it's no wonder that we run into these pale looking creatures every so often -- especially during the fall.
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Peltoperla nymph from 11/11/11.

By the way, though species ID is difficult with nymphs that have only recently molted, I suspect that our nymph from yesterday was Acroneuria abnormis.

No anal gills and no occipital setal row (= Acroneuria), and the posterior tergal margins appear to be light (= abnormis).